Peroxyacid manufacture

ABSTRACT

In one class of processes for making poorly soluble organic peroxyacids the corresponding carboxylic acid is reacted with hydrogen peroxide in a reaction medium containing a high concentration of sulphuric acid. The presence of such constituents in the reaction mixture and the manner of the reactants and the way in which they are brought into contact, can result in the processes being hazardous. In the present invention, hazard problems are reduced or eliminated by first dissolving the carboxylic acid in concentrated sulphuric acid, secondly forming a Caro&#39;s acid solution containing a complementary amount of sulphuric acid and hydrogen peroxide within a predetermined range, and then introducing the carboxylic acid solution at a controlled rate in the Caro&#39;s acid solution, often over a period of from about 30 to 90 minutes, with agitation and temperature control. The compositions of the two reactant solutions are precalculated relative to each other such that the A value (weight ratio of sulphuric acid to the combined weight of it and water) either increases during addition of the carboxylic acid solution or if the latter is relatively less insoluble, stays the same or increases. The solutions are preferably formulated to provide an A value at the end of the reaction period selected in the range of around 0.7 to around 0.8, at a temperature of around 20° to 45° C., the more soluble tending towards the lower end of each range and the less soluble towards the upper end of each range. The process is especially suitable for making poorly soluble aliphatic mono or diperoxycarboxylic acids such as peroxynonanoic acid or diperoxydodecanedioic acid, or for aromatic group substituted peroxyacids in which the peroxydation occurs in a primarily aliphatic environment, such as phthalimidoperoxyhexanoic acid.

The present invention relates to a process for the manufacture oforganic peroxyacids, and more particularly to organic peroxyacids havingpoor solubility in aqueous media.

The detergents industry constantly seeks to improve the cleansingperformance of its compositions, and in a effort to do so under lowtemperature washing conditions has investigated the incorporation ofquite small amounts of peroxyacids. Most of the peroxyacids contemplatedhave been organic peroxycarboxylic acid compounds containing at least 7carbon atoms and many of them have exhibited poor solubility in aqueousmedia, a feature usually shared with the corresponding carboxylic acidfrom which they are or could be derived.

There have been many different processes proposed for the manufacture oforganic peroxyacids, including the poorly soluble ones. A number haveused sulphuric acid as a main constituent of the reaction mixture.Parker et al in JACS 77:4037-41 and/or JACS 79:1929-32 suggested thedrop-wise addition of concentrated hydrogen peroxide to a sulphuric acidsolution of a long chain aliphatic mono or di-carboxylic acid. Hutchins,in U.S. Pat. No. 4,119,660, discloses in column 1 that there are anumber of problems with the Parker process, such as rate of peracidformation and/or small particle size of peracid product and accordingly,he suggests an alternative procedure in which the hydrogen peroxide andsulphuric acid reagents are premixed, and the solid carboxylic acid issubsequently introduced therein. Hutchins asserts that the acidity ofhis reaction mixture is of crucial importance, being at least 69%sulphuric acid in order to attain an unexpected decrease in reactiontime, compared even with an acidity of 66.5%. Virtually the sameprocedure had been proposed nearly 20 years earlier by Krimm in U.S.Pat. No. 2,813,896, the difference being that the exemplified amounts ofsulphuric acid were numerically lower than the range identified byHutchins. Camden in U.S. Pat. No. 4,233,235 discloses a continuousprocess for making similar long chain aliphatic peroxyacids and assertsin column 4 that batch processes, presumably for the same products,exhibit more safety problems and produce smaller crystals. In hisprocess, he employs balanced continuous addition of reagents to andcontinuous withdrawal of product a constant residence time of hisreagents in the reaction mixture. Bettle in U.S. Pat. No. 4,314,949discloses that in a preferred method of making aliphatic percarboxylicacids, particulate carboxylic acid is added to a peroxide/sulphuric acidmixture. Hutchins in a second patent, U.S. Pat. No. 4,244,884, disclosesa variation to the Camden continuous process, in which he recyclesmother liquor separated from the product. It will be observed that incolumn 3, he asserts that the carboxylic acid added to the reactionmixture typically precipitates in situ, so that most of it is in thesolid form. He also indicates that the inherent reaction betweencarboxylic acids and hydrogen peroxide can present safety problems,since it can become uncontrollable if it is not carefully controlled.

Peroxyacids are often needed in smaller quantities than justify thecapital outlay inherent in a continuously operated process.Consequently, it would be desirable to devise a batch process variationthat can be readily carried out so as to avoid the formation ofhazardous compositions during the entire process. In the course ofinvestigations which have lead to the instant invention, the inventorshave found that there are a number of different factors which must bebalanced in order to retain a viable and safe process.

A wide range of compositions containing relatively high concentrationsof hydrogen peroxide and organic material, such as carboxylic acids areconventionally viewed as hazardous on account of their tendency todecompose spontaneously. Thus, too much hydrogen peroxide in solutioncauses safety problems. Since the solubility of the carboxylic acid isusually rather low, it means that peroxide concentration must be keptlow by increase in the water and/or sulphuric acid contents. In thatcontext, the water content of the reaction mixture needs to be kept lowbecause it has been demonstrated by Hutchins in obtaining U.S. Pat. No.4,119,660 that the sulphuric acid content of the reaction is of veryconsiderable importance in achieving a fast rate of reaction andformation of a high molecular weight peroxyacid. This may be a way ofsaying that the reaction equation demonstrates the value of having verylittle water present, since the extent of formation of percarboxylicacid is the equilibrium mixture in the liquid phase is clearly affecteddirectly by residual water content.

    RCO.sub.2 H+H.sub.2 O.sub.2 =RCO.sub.3 H+H.sub.2 O

However, to set against the foregoing, the inventors have also foundthat the solubility of the product is very dependent upon the acidity ofthe reaction mixture, and increases very markedly as the acidityincreases. The inventors have further correlated such a finding with twoother observations. Both the proportionate recovery of the product, animportant factor in a batch process, and also the inherent degree ofsafety of operation of the process vary inversely to the acidity of theaqueous phase.

It is an object of the present invention to create a process which canbe operated in batch mode in a controlled and safe fashion and fromwhich can be recovered a crystalline peroxyacid product, preferably inhigh yield.

According to the present invention, there is provided a process for themanufacture of poorly-soluble aliphatic peroxyacids by reaction betweenan aliphatic carboxylic acid and excess hydrogen peroxide in a stronglyacidic reaction medium which is characterised in that the followingprocess sequence is employed:

1. In step 1, the aliphatic carboxylic acid is dissolved in concentratedaqueous sulphuric acid that contains no more than a maximum proportionof water which proportion varies in accordance with the solubility ofthe carboxylic acid from about 25% w/w water for the relatively moresoluble carboxylic acids to about 10% w/w water for the relatively lesssoluble carboxylic acids;

2. In step 2, an equilibrium mixture of Caro's acid is made by mixing i)hydrogen peroxide, ii) sulphuric acid and iii) water said mixturecontaining from about 5 to 30% hydrogen peroxide, a complementary amountof sulphuric acid such that, in conjunction with the amount of sulphuricacid present in the carboxylic acid solution of step 1, theconcentration of sulphuric acid in the reaction mixture at the end ofstep 3 is as defined therein, and the balance water, with agitation andcooling to below about 50° C., the A value, being the weight ratio ofsulphuric acid to the total of water plus sulphuric acid in acomposition, for the Caro's acid solution being not substantiallygreater than the A value of the carboxylic acid solution produced instep 1 or lower, the minimum difference between the two A valuesincreasing as the solubility of the carboxylic acid decreases;

3. In step 3, a reaction mixture is formed by introducing the product ofstep 1 with agitation into a body of Caro's acid formed in step 2 and ismaintained at a temperature in the range of from 0° to 50° C., the rateof introduction being controlled such that the aliphatic carboxylic acidremains substantially entirely in solution before it reacts withhydrogen peroxide or permonosulphuric acid and the introduction isstopped no later than when the composition of the aqueous phase hasreached a point selected within the ranges for i), ii) and iii) of about3 to 15% hydrogen peroxide, about 55 to 80% sulphuric acid, and thebalance being at least about 10% water, with the consequence that the Avalue of the aqueous phase of the reaction mixture in step 3 remainssubstantially constant or increases as the carboxylic acid solution isintroduced therein, the extent of the minimum increase in A value beinginversely related to the carboxylic acid solubility;

4. Solid peroxyacid product produced during step 3 is separated fromaqueous phase and retained as product.

Herein, for the purpose of determining how much of each of sulphuricacid, water and hydrogen peroxide is present in a composition, forexample the Caro's acid solution, the figures given represent therespective amounts of the three components that would be present if theequilibration reaction did not take place. Thus, for example, the figurefor sulphuric acid includes the proportion which in fact has beenconverted to peroxymonosulphuric acid as well as the proportion whichremained unconverted, and similarly the figure for hydrogen peroxideincludes the proportion which was also converted to peroxymonosulphuricacid.

By the use of the process according to the present invention, it ispossible to obtain reaction between the carboxylic acid and theperoxidising species, i.e. hydrogen peroxide or H₂ SO₅, without thecarboxylic acid passing out of solution. This makes the process simplerand safer to control because it avoids the variable of the carboxylicacid being present as a separate phase from the aqueous phase in whichthe desired reaction takes place. The resultant peroxycarboxylic acidhas been found to enjoy lower solubility than the carboxylic acid fromwhich it has been formed in the prevailing aqueous composition, andconsequently precipitates out of solution as it is formed.

As a result of so controlling the nature of the carboxylic acidfeedstock, in conjunction with the composition of the Caro's acid phase,the inventors have been able to provide a process which needs only acomparatively low sulphuric acid content during at least a majorproportion of the reaction time than has previously been suggested for adirectly comparable batch process. The manner of addition of thecarboxylic acid solution to the Caro's acid solution and the compositionof the two reactants means that any change in the composition of theaqueous phase in the reaction is constantly towards instead of throughhazardous compositions. Accordingly, the process can be easily halted,and indeed the process sequence is so designed that reagent additionterminates before a hazardous composition region is reached. Also, themanner of introduction of the carboxylic acid of the instant inventionmeans that the total amount of dissolved organic compounds is kept atvery low levels in the reaction mixture throughout the process whenperoxidising species are present. This again acts as a feature promotingsafe operation of the process.

Advantageously, the invention process also avoids the method of Parkeret al which comprised the introduction of hydrogen peroxide solutioninto a carboxylic acid/sulphuric acid. The Parker method not onlyproduces the rather small crystals according to its critic, Hutchins,but suffers far more seriously from the fact that the composition of theaqueous sulphuric acid phase inevitably passes into a region of extremehazard in or around about 85/90% sulphuric acid content as a result ofgradual addition of the hydrogen peroxide solution. It also maximisesthe amount of organic material that is present in the aqueous phase whena peroxidising species is present, which decreases safety during theoperation of the process. Whilst Parker's method can just becontemplated on a laboratory scale behind suitably protective screens,it is absolutely impractical for commercial plant operation.

Accordingly, it will be recognised that the present invention processcombines the advantages of operating with a preformed Caro's acidsolution with the advantages of employing the carboxylic acid in aliquid form.

The invention process is applicable to the formation of peroxyacid frompoorly water soluble aliphatic carboxylic acids. This term includes notonly compounds that contain simply a linear or branched hydrocarbonstructure that carries at least one carboxylic acid substituent, butadditionally includes derivatives in which a further substituent, suchas an aromatic group is sufficiently separated from the carboxylic acidsubstituent by the aliphatic hydrocarbon structure that it has nosignificant influence upon the peroxidation reaction, ie the startingmaterial is accepted as essentially aliphatic in the vicinity of thereaction point. Accordingly, the invention process encompasses asstarting material linear or branched aliphatic monocarboxylic acidscontaining from 8 to 12 carbon atoms, including nonanoic acid,iso-nonanoic acid, capric acid, and lauric acid, or mixtures of any twoor more thereof. Alternatively, the starting material may comprisealiphatic dicarboxylic acids, often alpha,omega dicarboxylic acidscontaining from 6 to 16 carbon atoms, including suberic acid, azelaicacid, 1,10-decanedioic acid, 1,12-dodecanedioic acid,1,14-tetradecanedioic acid and 1,16-hexadecanedioic acid.

In a third and potentially interesting variation, the starting materialcan comprise compounds of general formula R°--A--CO₂ H in which Arepresents a hydrocarbon diradical separating the substituent R° fromthe carboxylic acid carbon atom by at least 3 and often from 3 to 8carbon atoms linearly, and R° represents a non-aliphatic substituent. R°can be an unsubstituted aromatic such as phenyl, or be substituted by anon-interfering substituent such as an alkyl or halo group, eg methyl,tertiary butyl, chloro or bromo or can comprise an aromatic amide orimide group, ie have one of the formulae: ##STR1## in which Arpreferably represents a phenyl group. It will be recognised that thephenyl group itself may be substituted by an alkyl group, such ascontaining up to 12 carbon atoms, and/or by a second amidoalkanoic acidor imidoalkanoic acid group. The process has particular applicability tophthalamidoalkanoic acids and phthalimidoalkanoic acids containing from4 to 7 linear carbon atoms in the alkanoic acid group, such asphthalimidohexanoic acid, which hereinafter may be alternativelyreferred to as PICA for convenience.

Alternatively, in a fourth variation similar to variation 3, thestarting material is also an amido or imidoalkanoic acid of formulaR°--A--CO₂ H in which A represents a hydrocarbon diradical separatingthe substituent R° from the carboxylic acid carbon atom by at least 2and often up to 8 carbon atoms linearly, and R° represents an aliphaticsubstituent satisfying the sub-formula: ##STR2## in which Ak representsan alkyl group containing at least 6 carbons or a dimethylene group,optionally alkyl substituted or forming part of a cycloaliphaticnucleus. Representative compounds include octanamido ornonanamidosuccinic acid succinimidobutyric acid orhexahydrophthalimidocaproic acid.

Within the broad ambit of the present invention, it will be recognisedthat there are a number of variables under the control of the processoperator. These include, in particular, the composition of thecarboxylic acid solution, the composition of the Caro's acid solution,the overall weight ratio of the two solutions employed and thetemperature of the reaction mixture in step 3.

It must be understood that the inherent solubility of the carboxylicacid starting material is of considerable influence upon the selectionof the operating values for the variables within the broad limits. As ageneral guideline, there is a preferred operating window of conditionsfor each starting material that is positioned in accordance with theinherent solubility of the carboxylic acid in aqueous sulphuric acidliquors. By way of general guidance, carboxylic acid starting materialswhich have solubility towards the upper end of the solubility range,such as PICA, tend to benefit from employing less stringent operatingconditions than carboxylic acids which have a solubility towards thelower end of the solubility range such as dodecanedioic acid, sometimesreferred to herein as DDA. In this context, the term "stringent" impliesin particular the presence of a greater proportion of sulphuric acid inthe solution in which the carboxylic acid is dissolved in step 1 oralternatively (or additionally) the use of a higher ratio of carboxylicacid solution produced in step 1 to the Caro's acid solution produced instep 2. The use of more stringent conditions results in the reactionmixture containing a higher suphuric acid content than if less stringentconditions had been employed. In addition, more stringent conditions arepreferably operated in conjunction with a higher operating temperature.The subsequent description of preferred embodiments should accordinglybe read in the light of the foregoing generalisations.

The range of sulphuric acid concentrations that it is practical toemploy depends upon which carboxylic acid is being employed. It ispractical to use sulphuric acid of at least 90% strength and sometimesmost convenient and preferably to employ sulphuric acid of at least 95%w/w as the solvent for carboxylic acids of similar solubility to DDA, iecarboxylic acids of inherently poorer solubility. It is also practicalto employ such sulphuric acid concentrations for the slightly moresoluble carboxylic acid reagents than DDA, but as the inherentsolubility of the carboxylic acid increases, it becomes increasinglypractical to select a lower strength sulphuric acid as solvent. Thus,for a compound like PICA, it is practical, from some points of view, toemploy in step 1 a sulphuric acid concentration as low as around 80%.

The solutions can be made in step 1 readily by mixing the two componentsat a suitable temperature to promote the dissolution process, andpreferably under enclosed conditions so as to prevent or minimise theloss or particles of carboxylic acid into the atmosphere. Either ambientor elevated temperature solutions can be produced, preferably notexceeding 60° C. It is particularly convenient for the solution after ithas been made to have a temperature that does not exceed the temperatureadopted in step 3, although to accelerate dissolution of the carboxylicacid, the sulphuric acid solvent may be heated during the dissolutionand subsequently cooled or allowed to cool to the desired temperaturefor its introduction into the reaction mixture. It will be seen that thetechnique enables the process operator to avoid the use of finely groundsolid particles advocated by Bettle in his above-identified US patentspecification, and thereby avoids the dust and hazard implications ofBettle's process. It will be further recognised that such beneficialconditions would be significantly more risky if the dissolution were totake place in the presence of peroxidising species, ie in a manner thatis not according to the instant invention.

The composition of the Caro's acid solution made in step 2 is decided inconjunction with the composition of the sulphuric acid solvent for thecarboxylic acid so that the total amount of sulphuric acid provided byboth compositions is appropriate for the selected carboxylic acidstarting material and peroxyacid product. Where the one compositionprovides a relatively high amount of sulphuric acid, then the othercomposition tends to provide a correspondingly low amount of sulphuricacid, but viewed in the light of the overall requirement of the startingcarboxylic acid material.

One other and implicit factor taken into account when determining theactual concentration of sulphuric acid in the Caro's acid to provide thecomplementary amount is the overall volume of liquor in the reactionmixture at the end of introduction of the carboxylic acid solution instep 3 relative to the amount of percarboxylic acid solids. Where theratio of the two is low, the concentration of sulphuric acid in theCaro's acid is also relatively low and vice versa in order to attain thesame sulphuric acid concentration in the aqueous phase of the reactionmixture at the end of the introduction of the carboxylic acid solution.The resultant difference in the strength of the Caro's acid solution isnaturally more pronounced when the highest strength sulphuric acidconcentration is employed for dissolving the carboxylic acid.

The concentration of sulphuric acid in the Caro's acid solution isnormally selected in the range of 5 to 70% w/w, is often at least 35%w/w and commonly in the range 40 to 65% w/w.

For the most poorly soluble starting materials like DDA or even higherweight dicarboxylic acids, it is desirable in some embodiments, thoughnot essential to employ Caro's acid solutions towards the upper end ofthe range as regards its sulphuric acid content. By way of illustration,a concentration of around 55 to 65% w/w sulphuric acid is convenient,together with hydrogen peroxide content of preferably around 10 to 20%,especially 12 to 18% and water providing the residue. For the lesspoorly soluble carboxylic acid starting materials like PICA oralkylamidosuccinic acids, in other embodiments, it is often convenientto start with a composition containing somewhat lower amounts ofsulphuric acid than indicated for DDA, such as an intermediate range offrom 45 to 55% w/w or even 40-45% w/w, and a correspondingly higheramount of water, so that advantage can be taken of employing arelatively lower sulphuric acid concentration in the reaction mixture.The hydrogen peroxide content is chosen preferably within the range 10to 20% and especially 12 to 18% w/w. Advantage can therefore be takenfor compounds like PICA to be peroxidised under conditions even furtherfrom hazardous regions.

However, in yet other embodiments, it is possible to employ intermediaterange strength Caro's acid solutions in conjunction with any of thecarboxylic acids, provided that the total amount of sulphuric acid inthe reaction mixture in step 3 is sufficiently high for theperoxycarboxylic acid to precipitate out. Such a process variation ispreferably operated in conjunction with either an extended period ofintroduction of the carboxylic acid solution into the reaction mixtureand/or an extended post-introduction or digestion phase. For use inconjunction with the more insoluble acids, the sulphuric acidconcentration in the Caro's acid solution is usually at least 35% w/w.Progressively, the practical lower limit for sulphuric acid strengthfalls below 35% as the solubility of the carboxylic acid becomes higher.

For convenience herein, reference is made from time to time to the term"A value" of an aqueous composition by which is meant the fractionobtained by dividing the sulphuric acid weight therein, S, by the sum ofthe weight of sulphuric acid and water therein, [S+W]. An importantfactor in the invention process comprises the difference in strength ofthe sulphuric acid solutions used in steps 1 and 2, which can beexpressed as the difference in A value (ΔA) between the two reagentsolutions, A₁ -A₂. The minimum and maximum difference in ΔA value isrespectively about 0 and 0.9. The practical breadth of the range that isuseable varies in line with the solubility of the carboxylic acid beingperoxidised. For carboxylic acids like dodecanedioic acid A₁ normallyexceeds A₂ by at least 0.2, and is normally less than about 0.6,sometimes in the range 0.2 to 0.3 but sometimes also from 0.3 to about0.5. As the solubility of the acid increases to or beyond that of PICAthe practical range for ΔA broadens, the minimum ΔA for practicalworking reducing towards 0 and the maximum ΔA increasing towards 0.9.

During step 2, the formation of Caro's acid is strongly exothermic, andaccordingly, the composition is normally cooled by the provision of acooling jacket or coils through which a cold fluid is pumped or bypassage through a cooling heat exchanger. Conveniently, the cooling isso controlled as to produce a temperature at or similar to the processtemperature of step 3. It will be recognised as a benefit of the instantprocess that a significant proportion of the heat inherent in conductinga peroxidation in a sulphuric acid/hydrogen peroxide reaction medium canbe generated and removed prior to the organic compounds being present,thereby minimising the risks of a self-accelerating decompositionprocedure being set in train, inadvertantly.

The temperature of the reaction mixture is preferably maintained withina sub-range of the broad range which varies inversely to the relativesolubility of the carboxylic acid in aqueous sulphuric acid mixtures. Asthe relative solubility increases from the very poorly soluble, like DDAto the less poorly soluble, like PICA, the preferred reactiontemperature decreases from the sub-range of 35° to 45° C. to thesub-range of 15° to 30° C. Maintenance of mixture in the preferredtemperature sub-range assists in promoting an effective balance ofcrystal nucleation and growth of the selected peroxyacid, to attain aproduct which can be recovered more easily and a reaction mixture ofreasonable viscosity.

One of the important aspects of the present process resides in step 3,namely controlling the rate of introduction of the carboxylic acidsolution into the body of Caro's acid solution. Qualitatively, the rateis slow and progressive, by which latter term is meant that solution isintroduced as a stream or in the form of extremely small incrementswhich for practical purposes is in essence like a stream. The rate iscontrolled so as to prevent the carboxylic acid precipitating out whenit encounters the body of Caro's acid solution. A suitable rate cannaturally be established for each starting material under the prevailingconditions by small scale tests and prior observation of the solubilityprofile for the starting material. The most preferred rate from theviewpoint of maximising through-put is that which is virtually borderingupon the rate at which carboxylic acid would begin to precipitate out inany significant extent. It will be recognised of course that such a ratewill be dependent upon the temperature, and the composition of theaqueous phase as well as the nature of the carboxylic acid compositionthat is introduced.

The rate of introduction of the carboxylic acid solution in step 3 maybe kept constant during the entire introduction of the carboxylic acid,at its initial rate, but in a preferred variation the rate is increasedas the reaction progresses. The increase tends to be least in theinitial stages and accelerates towards the end of the reaction. Thus,when so operated, the rate tends to follow the change in sulphuric acidcontent of the aqueous phase. It will also be seen that the increase inaddition rate tends to enable cooling equipment to be matched better tothe plant capacity. Initially, the difference between the Caro's acidbody of fluid and the carboxylic acid solution is at its greatest, sothat the heat of dilution is at its greatest. Later on, when thedifference in composition is smaller, so that the heat of dilution iscorrespondingly smaller per unit addition of carboxylic acid solution,the rate of introduction of the solution is greater so that there is atendency to balance the two effects. This enables the process user tooptimise the size of his cooling capacity.

A further advantage of the manner of introduction of the carboxylic acidmanifests itself in the convenient rate of nucleation and smoothdeposition of peroxyacid product, thereby forming especially underpreferred core operating conditions a relatively large and readilyfiltered crystalline product.

The total period for introducing the carboxylic acid solution is oftenselected within the range of from 30 to 200 minutes, the preferredsection of the range depending upon the solubility of the carboxylicacid under the prevailing conditions, the better the solubility, theshorter the permissible introduction period. As has been referred tohereinbefore, solubility increases with both increase in sulphuric acidcontent and increase in temperature. To some extent, at least, arelative decrease in inherent solubility can be compensated by anincrease in reaction temperature. The period of introduction is oftenselected in the range of from 45 to 90 minutes at a preferred reactionmixture temperature in step 3 of from 15° to 30° C. for acids like PICA,and increased for acids like DDA, to a preferred reaction mixturetemperature of from about 35° to 45° C. It is of course suitable toemploy a slower rate of introduction of the carboxylic acid solution,such as selected in the range of 90 to 150 minutes, but at the expenseof reduced through-put. From the product quality stand point, though, itis often at least as good as at the slightly faster rate ofintroduction.

Whilst the reaction process may be terminated as soon as all thecarboxylic acid feedstock has been introduced, it is preferable to allowa further period in which the reaction can progress more fully tocompletion and if desired to allow some digestion of the crystallineproduct. This is particularly desirable if ΔA is in the region of atleast 0.4. A convenient post-introduction period is often up to about150 minutes and in some instances is from 10 to 60 minutes. In otherinstances when ΔA was high, it can conveniently comprise 60 to 120minutes. Thus, a convenient combined period for a starting material likePICA comprises about 80 to 100 minutes at around ambient temperaturereaction, though it may sometimes last from 100 to about 150 minutes.

The total amount of carboxylic acid solution to be introduced per unitvolume of Caro's acid solution will depend, of course, upon the actualcomposition of each, and especially upon the concentration of carboxylicacid in the sulphuric acid solvent. In many instances the carboxylicacid concentration will be chosen within the range of from 20 to 40%w/w, depending upon the solubility of the material and the viscosity ofthe resultant solution. At the lower end of the carboxylic acidconcentrations, e.g. 20 to 25% solutions, the carboxylic acid solution:Caro's acid weight ratio will be commonly selected within the range offrom 1.5:1 to 1:1.7. This can be particular suitable for acids likePICA. As the carboxylic acid concentration increases, the ratio tilts infavour of more Caro's acid, towards the ratio range of from 1:1.8 toabout 1:2.5 at a carboxylic acid concentration of about 35%. The overallrange span is therefore normally from 1.5:1 to 1:2.5. In manyembodiments roughly half, say 40 to 60% of the total sulphuric acid isintroduced as solvent for the carboxylic acid and the balance in theCaro's acid solution. In some other embodiments in which ΔA is high, theproportion of sulphuric acid in the carboxylic acid solution is ratherhigher, and in the range of 60 to 75%. Either way, in view of theconcentration of hydrogen peroxide in the caro's acid solutionpreferably comprising at least 10% w/w, it means that the amount ofhydrogen peroxide employed is normally significantly in excess of thestoichiometric amount.

The aqueous phase of the reaction mixture normally contains a higherweight fraction and in many embodiments a substantially higher weightfraction of sulphuric acid at the end compared with the start of step 3,depending upon ΔA. Whilst it would be feasible, in those embodiments inwhich there is a significant increase in sulphuric acid fraction, tochoose to operate by introducing the carboxylic acid solution until apredecided maximum sulphuric acid content is attained, the actual pointbeing selected in the light of the solubility of the carboxylic acid andthe percarboxylic acid, it is often more convenient to prepare in steps1 and 2 the appropriate amounts of the two solutions so as to attainthat sulphuric acid content when they are fully mixed together.

The target proportions of sulphuric acid and water in the aqueous phaseof the reaction mixture at the end of the reaction period are chosennormally in the light of the solubility characteristics of thecarboxylic acid reactant and peroxycarboxylic product and normallywithin the ranges of about 55 to 80% w/w sulphuric acid and about 10 to40% water. For a product like phthalimidoperoxyhexanoic acid, the finalsulphuric acid content is often selected in the range of about 60 to 67%w/w in the aqueous phase and in addition, which corresponds to an Avalue that is often from 0.7 to 0.75. For the production of relativelysoluble peroxyacids, such as p-chloroperoxyadipanilic acid, it can beconvenient for the final sulphuric acid content to likewise fall in therange of 60-67% w/w but advantageously, it can fall below 60% w/w, suchas from about 55 to 60% w/w and the selected final A value wouldaccordingly be less than 0.7, such as from 0.6 to 0.7.

For less soluble peroxyacids, such diperoxydodecanedioic acid thepreferred range of A value in the reaction mixture at the end of step 3tends to overlap at its lower end with the upper end of the range formore soluble peroxyacids, from about 0.72 to about 0.8 and especially upto about 0.76. This means a correspondingly higher preferred range ofresidual proportions of sulphuric acid in the reaction mixture,approximating to about 65 to 75% w/w, and a corresponding content ofwater towards the lower end of its range, such as from 10 to 25%. The Avalue changes by virtue of the introduction of the higher strengthsulphuric acid in the carboxylic acid solution. This could, if desired,be augmented by a separate addition of concentrated sulphuric acid, orby diluting the carboxylic acid solution with extra sulphuric acid.

The reaction in step 3 can be carried out in the presence of additionalsolids material, which in practice is normally an extra amount of theperoxyacid, such as produced in a previous batch. The amount of suchadditional solids is often selected in the range of from 0 to 10% w/wbased upon the weight of the reaction mixture. Its presence can promotea larger average particle size for the product.

The reaction can be carried out in conventional reaction vessels ortanks equipped with means to thoroughly agitate the mixture, preferablyavoiding excessive shear so as to minimise fragmentation of the productcrystals. The vessels and pipework should preferably be made frommaterials that are resistant to corrosion from aqueous sulphuric acidsolutions, such as from appropriate grades of steel or be lined with asuitably resistant elastomeric lining.

The particulate peroxyacid product is separated from the aqueous phaseat the end of the reaction. Conventional solid/liquid separating devicescan be used, including filters or centrifuges. By virtue of the way inwhich the product is made, we have found that it has only a low residualcarboxylic acid content. Thus, in many instances, not only is theproduct rather pure, but a high conversion to the percarboxylic acid isachieved.

The solid phase is retained as the product, and usually contains asubstantial weight of aqueous phase. Since sulphuric acid impurity tendsto impair the stability of peroxycarboxylic acids, it is preferable towater-wash and dry the product. Alternatively, or additionally, anddepending upon the inherent safety of the peroxyacid, it can bedesirable to effect desensitisation of the peroxyacid before it isallowed to dry out. This can be effected by known techniques, such asthe in situ partial neutralisation of the entrained sulphuric acid witha suitable alkali such as a sodium or magnesium hydroxide and/or bymixing the damp product with a desensitiser such a boric acid or sodiumor magnesium sulphate that is prepared beforehand. Such desensitisationis particularly desirable for the diperoxyalkanoic acids like DPDDA, butcan often be unnecessary for the much safer compounds likephthalimidoperoxyhexanoic acid or phthaliamidoperoxyhexanoic acid.

The mother liquor contains a significant content of hydrogenperoxide/permonosulphuric acid. In order to improve the economics of theprocess, the mother liquor can be recycled at least in part after itscomposition has been adjusted to approximately that of the Caro's acidsolution produced in step 2. Such adjustment is made by diluting themother liquor with water to reduce the A value and concentrated hydrogenperoxide solution to increase the residual hydrogen peroxide level. Thedilutions can be sequential or simultaneous. In practice, it is oftennot possible to recycle the entire volume of mother liquor because theretained amount of liquor in general and sulphuric acid in particular istoo great. In such circumstances, the loss of a fraction of the motherliquor acts as a means to remove byproducts from the reaction, and inparticular degradation products of the carboxylic acid and therebyreduce the rate at which they would build-up during multiple recycle ofthe mother liquor. Periodically, the entire batch of mother liquor canbe discarded, if desired.

If desired, the mother liquor can be diluted with an aqueous medium,such as water itself or aqueous hydrogen peroxide solution, such asenough for recycling or an excess amount. The net result of dilution isto reduce the solubility of the peroxyacid product, and thereby cause afurther amount of precipitate to form, which can subsequently berecovered. An alternative or additional means for reducing theperoxyacid solubility comprises cooling the liquor, preferably to belowabout 10° C. or by at least 10° C. below the reaction temperature ofstep 3. Both procedures minimise the amount of carboxylic acid andperoxyacid product that is recycled, so that the extent of theirdegradation is also minimised. By reducing the residual content oforganic species in solution, recycling the mother liquor is rendered aless hazardous procedure.

The peroxycarboxylic acids produced by a process according to theinstant invention can be employed for the various known uses for suchcompounds, including incorporation in fabric bleaching or washingcompositions, and especially those intended for operation at hand hottemperatures or lower, in disinfectant or sanitizing compositions forsolid surfaces, liquid media or gasses, or as a reagent for oxidisingreactions or for polymerisation or cross linking ethylenicallyunsaturated materials.

Having described the invention in general terms, certain embodimentsthereof will now be described more fully by way of example only.

EXAMPLE 1

Phthalimidohexanoic acid, 30 g, was dissolved in sulphuric acidsolution, 80% w/w, 90 g, at 20° C. A Caro's acid solution was preparedby mixing with cooling to about 20° C. water, concentrated hydrogenperoxide and concentrated sulphuric acid to provide 90 g of a solutioncontaining 50% w/w sulphuric acid, 20% w/w hydrogen peroxide and 30% w/wwater, which is equivalent to an A value of 0.625.

The solution of phthalimidohexanoic acid was then introduced into Caro'sacid solution at a rate of 2 g per minute with constant stirring andcooling to 20° C. The introduction was complete after 1 hour, and thereaction mixture, which had the form of a thick slurry, was stirred fora further 30 minutes. The mixture was filtered and the solids washedwith three portions, each of about 200 ml of demineralised water,yielding 26.1 g of a white crystalline product which analysis confirmedas phthalimidoperoxyhexanoic acid at a purity of 96%. The mother liquorwas retained for recycle.

EXAMPLE 2

Phthalimidohexanoic acid, 30 g, was dissolved in sulphuric acidsolution, 98% w/w, 40.5 g, at 20° C. A Caro's acid solution was preparedby mixing with cooling to about 20° C. water, concentrated hydrogenperoxide and concentrated sulphuric acid to provide 130.5 g of asolution containing 50% w/w sulphuric acid, 20% w/w hydrogen peroxideand 30% w/w water, which is equivalent to an A value of 0.625.

The solution of phthalimidohexanoic acid was then introduced into Caro'sacid solution at a rate of 1.18 g per minute with constant stirring andcooling to 20° C. The introduction was complete after 1 hour, and thereaction mixture, which had the form of a thick slurry, was cooled to10° C. and stirred for a further 1 hour. The mixture was filtered andthe solids washed with three portions, each of about 200 ml ofdemineralised water, yielding 23.8 g of a white crystalline productwhich analysis confirmed as phthalimidoperoxy-hexanoic acid at a purityof 99.9%. The mother liquor was retained for recycle.

EXAMPLE 3

In Example 3, dodecanedioic acid, 50 g, was dissolved in sulphuric acidsolution, 96.4% w/w, 116.7 g, at 20° C. A Caro's acid solution, 378.25g, containing 253.4 g H₂ SO₄, 39.7 g H₂ O₂ and 85.1 g H₂ O (A value0.749) was prepared by mixing concentrated sulphuric acid, water andconcentrated hydrogen peroxide with cooling to 45° C.

The dodecanedioic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 2.78 g per minute for aperiod of 1 hour, the reaction being maintained at 45° C. The mixturewas stirred for a further 30 minutes at the same temperature, filteredand the solids washed with water. The resultant product, 55 g, had apurity of 95.3% diperoxydodecanedioic acid.

Example 3 was repeated, (Example 3A) but employing a Caro's acidsolution which contained 221.2 g H₂ SO₄, 51.7 g H₂ O₂ and 105.3 g H₂ O,(A value 0.677). Substantially the same recovery and purity wasobtained.

Example 3A, which is preferred, demonstrates that product quality can bemaintained by following the instant process, whilst reducing the A valueof the reagent and the reaction mixture, so that at no time, even at theend of the reaction when all the sulphuric acid has been added does themixture an unsafe or meta-safe composition.

EXAMPLE 4

In Example 4, lauric acid, 20 g, was dissolved in concentrated sulphuricacid solution, 98.0% w/w, 60 g at ambient temperature. A Caro's acidsolution, 60 g, containing 37% w/w H₂ SO₄, 20% w/w H₂ O₂ and 43% w/w H₂O (A value 0.463) was prepared by mixing concentrated sulphuric acid,water and concentrated hydrogen peroxide with cooling to about 20° C.

The lauric acid solution was introduced with stirring into the Caro'sacid solution continuously at a rate of 1.6 g per minute for a period of50 minutes, the reaction mixture being maintained at 35°-40° C. At theend of the addition the solution was clear. The mixture was stirred fora further 2 hours at about 40° C., during which period a crystallineproduct precipitated. Solids were recovered from the mixture by thefollowing method, hereinafter SRT for short, in which a) the mixture wascooled to about 10° C., b) quenched by mixture with an approximatelyequal volume of ice/water, the solid product was washed with water untilthe filtrate had a pH of about pH5 and air dried. The yield was 24g,having a purity of 71.3% peroxylauric acid.

EXAMPLE 5

In Example 5, nonanoic acid, 20 g, was dissolved in concentratedsulphuric acid solution, 98.0% w/w, 60 g at ambient temperature. ACaro's acid solution, 60 g, containing 37% w/w H₂ SO₄, 20% w/w H₂ O₂ and43% w/w H₂ O (A value 0.463) was prepared by mixing concentratedsulphuric acid, water and concentrated hydrogen peroxide with cooling toabout 20° C.

The carboxylic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 2.67 g per minute for aperiod of 30 minutes, the reaction being maintained at 25°-30° C. Themixture was stirred for a further 1.5 hours at 30° C., and subjected toSRT. The resultant product, 25.7 g, had a purity of 76.2% pernonanoicacid.

EXAMPLE 6

In Example 6, phthalimidobutyric acid, 20 g, was dissolved inconcentrated sulphuric acid solution, 98.0% w/w, 60 g at ambienttemperature. A Caro's acid solution, 60 g, containing 37% w/w H₂ SO₄,20% w/w H₂ O₂ and 43% w/w H₂ O (A value 0.463) was prepared by mixingconcentrated sulphuric acid, water and concentrated hydrogen peroxidewith cooling to about 20° C.

The carboxylic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 2.67 g per minute for aperiod of 30 minutes, the reaction being maintained at 20° C. Themixture was stirred for a further 90 minutes at the same temperature,and solids recovered by the SRT method. The resultant product, 18.5 g,had a purity of 94.3% phthalimdoperoxybutyric acid.

EXAMPLE 7

In Example 7, p-chloroadipanilic acid, 20 g, was dissolved inconcentrated sulphuric acid solution, 98.0% w/w, 60 g at ambienttemperature. A Caro's acid solution, 60 g, containing 6% w/w H₂ SO₄,26.6% w/w H₂ O₂ and 67.4% w/w H₂ O (A value 0.082) was prepared bymixing concentrated sulphuric acid, water and concentrated hydrogenperoxide with cooling to ambient temperature.

The carboxylic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 2.67 g per minute for aperiod of 30 minutes, the reaction being maintained at 20° C. Themixture was stirred for a further 2 hours at the same temperature, andsubjected to the SRT method. The resultant product, 20.1 g had a purityof 91.0% chloroperoxyadipanilic acid.

EXAMPLE 8

In Example 8, 6,6'-terephthal-di(amidohexanoic) acid, 20 g, wasdissolved in concentrated sulphuric acid solution, 98.0% w/w, 80 g atambient temperature. A Caro's acid solution, 83.5 g, containing 37.4%w/w H₂ SO₄, 20.8% w/w H₂ O₂ and 41.8% w/w H₂ O (A value 0.472) wasprepared by mixing concentrated sulphuric acid, water and concentratedhydrogen peroxide with cooling to ambient temperature.

The carboxylic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 3.33 g per minute for aperiod of 30 minutes, the reaction being maintained at 20°-25° C. Themixture was stirred for a further 2 hours minutes at 30° C., andsubjected to the SRT method. The resultant product, 19.9 g, had a purityof 89.0% 6,6'-terephthal-di(amidoperoxyhexanoic) acid.

EXAMPLE 9

In Example 9, 6,6'-fumaryl bis(amidohexanoic) acid, 20 g, was dissolvedin concentrated sulphuric acid solution, 98.0% w/w, 80 g at ambienttemperature. A Caro's acid solution, 80 g, containing 39% w/w H₂ SO₄,17.3% w/w H₂ O₂ and 43.7 w/w H₂ O (A value 0.472) was prepared by mixingconcentrated sulphuric acid, water and concentrated hydrogen peroxidewith cooling to ambient.

The carboxylic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 3.33 g per minute for aperiod of 30 minutes, the reaction being maintained at 30° C. Themixture was stirred for a further 2 hours at 40° C. and then subjectedto SRT. The resultant product, 16.9 g had a purity of 70% 6,6'-fumarylbisamidoperoxyhexanoic acid.

EXAMPLE 10

In Example 10, nonanamidosuccinic acid, 15 g, was dissolved inconcentrated sulphuric acid solution, 98.0% w/w, 45 g at ambienttemperature. A Caro's acid solution, 45 g, containing 30% w/w H₂ SO₄,30% w/w H₂ O₂ and 40% w/w H₂ O (A value 0.423) was prepared by mixingconcentrated sulphuric acid, water and concentrated hydrogen peroxidewith cooling to ambient temperature.

The carboxylic acid solution was introduced with stirring into theCaro's acid solution continuously at a rate of 3 g per minute for aperiod of 20 minutes, the reaction being maintained at 20°-25° C. Themixture was stirred for a further 1 hour at the same temperature, andthen subjected to the SRT. The resultant product, had a purity of 86.0%nonanamidoperoxysuccinic acid.

We claim:
 1. A process for the manufacture of poorly-soluble aliphaticperoxyacids by reaction between an aliphatic carboxylic acid and excesshydrogen peroxide in a strongly acidic reaction medium which ischaracterised in that the following process sequence is employed:in step1, the aliphatic carboxylic acid is dissolved in aqueous sulphuric acidthat contains no more than a maximum proportion of water whichproportion varies in accordance with the solubility of the carboxylicacid from about 25% w/w water for the relatively more soluble carboxylicacids to about 10% w/w water for the relatively less soluble carboxylicacids; in step 2, an equilibrium mixture of Caro's acid is made bymixing i) hydrogen peroxide, ii) sulphuric acid and iii) water saidmixture containing from about 5 to 30% hydrogen peroxide, an amount ofsulphuric acid such that, in conjunction with the amount of sulphuricacid present in the carboxylic acid solution of step 1, theconcentration of sulphuric acid in the reaction mixture at the end ofstep 3 is as defined therein, and the balance water, with agitation andcooling to below about 50° C., the A value, being the weight ratio ofsulphuric acid to the total of water plus sulphuric acid in acomposition, for the Caro's acid solution being not substantiallygreater than the A value of the carboxylic acid solution produced instep 1 or lower, the minimum difference between the two A valuesincreasing as the solubility of the carboxylic acid decreases; in step3, a reaction mixture is formed by introducing the product of step 1with agitation into a body of Caro's acid formed in step 2 and ismaintained at a temperature in the range of from 0° to 50° C., the rateof introduction being controlled such that the aliphatic carboxylic acidremains substantially entirely in solution before it reacts withhydrogen peroxide or permonosulphuric acid and the introduction isstopped no later than when the composition of the aqueous phase hasreached a point selected within the ranges for i), ii) and iii) of about3 to 15% hydrogen peroxide, about 55 to 80% sulphuric acid, and thebalance being at least about 10% water, with the consequence that the Avalue of the aqueous phase of the reaction mixture in step 3 remainssubstantially constant or increases as the carboxylic acid solution isintroduced therein, the extent of the minimum increase in A value beinginversely related to the carboxylic acid solubility; in step 4, solidperoxyacid product produced during step 3 is separated from aqueousphase and retained as product.
 2. A process according to claim 1characterised in that the carboxylic acid is selected from linear orbranched aliphatic monocarboxylic acids containing from 8 to 12 carbonatoms, or aliphatic alpha, omega dicarboxylic acids containing from 6 to16 carbon atoms or from aromatic substituted aliphatic carboxylic acidsof formula R°--A--CO₂ H in which A represents a hydrocarbon diradical of3 to 8 carbon atoms linearly and R° represents an aromatic group or agroup of formula ##STR3## in which Ar is an aromatic group or an amidoor imidoalkanoic acid of formula R°--A--CO₂ H in which A represents ahydrocarbon diradical separating the substituent R° from the carboxylicacid carbon atom by at least 2 up to 8 carbon atoms linearly, and R°represents an aliphatic substituent satisfying the sub-formula: ##STR4##in which Ak represents an alkyl group containing at least 6 carbons or adimethylene group, optionally alkyl substituted or forming part of acycloaliphatic nucleus.
 3. A process according to claim 2 characterisedin that the carboxylic acid is dodecanedioic acid.
 4. A processaccording to claim 2 in which the carboxylic acid is phthalimidohexanoicacid.
 5. A process according to any of claims 1, 2 or 4 characterised inthat the carboxylic acid is dissolved in sulphuric acid of at least 95%w/w strength in step
 1. 6. A process according any of claims 1, 2 or 4characterised in that the carboxylic acid which is less insoluble thandiperoxydodecanedioic acid in aqueous sulphuric acid mixtures isdissolved in step 1 in 80 to 85% w/w strength sulphuric acid.
 7. Aprocess according to any of claims 1, 2 or 4 characterized in that themixture of water, hydrogen peroxide, and sulfuric acid in step 2contains from 5 to 70% w/w sulfuric acid.
 8. A process according toclaim 7 characterized in that the mixture of water, hydrogen peroxideand sulfuric acid in step 2 contains at least 35% w/w sulfuric acid. 9.A process according to claim 8 characterized in that the composition ofthe mixture of sulfuric acid, hydrogen peroxide and water formed in step2 is produced within ranges which vary with the solubility of thecarboxylic acid in aqueous sulphuric acid mixtures, varying from acomposition of sulphuric acid 40 to 55%, hydrogen peroxide 10 to 20% andwater 25 to 45% w/w for the less insoluble carboxylic acids up to acomposition of sulphuric acid 55 to 65%, hydrogen peroxide 10 to 20% andwater 25 to 35% for the more insoluble carboxylic acids.
 10. A processaccording to any of claims 1, 2 or 4 characterised in that the reactionmixture produced by introduction of all the carboxylic acid solutioninto the Caro's acid solution has a composition which is selected withina window of compositions the position of which varies in accordance withthe solubility of the resultant peroxycaboxylic acid, and which has an Avalue of from about 0.7 to about 0.75 for the less insoluble peroxyacidsand the range changes to an A value of about 0.72 to about 0.8 for themore insoluble peroxyacids.
 11. A process according to any of claims 1,2 or 4 characterised in that the reaction mixture is maintained duringstep 3 at a temperature which is selected in accordance with thesolubility of the carboxylic acid in aqueous sulphuric acid mixture, thetemperature sub-range of 15° to 30° C. for the less insoluble carboxylicacids varying to the sub-range of 35° to 45° C. for the more insolublecarboxylic acids.
 12. A process according to claim 11 characterised inthat the carboxylic acid solution is introduced into the Caro's acidsolution during a period selected from 30 to 90 minutes.
 13. A processaccording to any of claims 1, 2 or 4 characterised in that thecarboxylic acid solution is introduced into the Caro's acid solution ata rate which is accelerated during the course of the reaction.
 14. Aprocess according to claim 3 characterized in that the carboxylic acidsolution formed in step 1 has a concentration of 20 to 40% w/wdodecandioic acid in at least 95% strength sulphuric acid and wherein,in step 2, said carboxylic acid solution is introduced into a Caro'sacid solution having a composition of sulphuric acid 55 to 65%, hydrogenperoxide 10 to 20% and water 25 to 35% at a temperature maintained atfrom 35° to 45° C. to form a final composition having an A value in therange of 0.72 to 0.8.
 15. A process according to claim 4 characterizedin that the carboxylic acid solution formed in step 1 has aconcentration of 20 to 35% w/w phthalimidohexanoic acid in at least 80%strength sulphuric acid and wherein, in step 2, said carboxylic acidsolution is introduced into a Caro's acid solution having a compositionof sulfuric acid 40 to 55%, hydrogen peroxide 10 to 20% and water 25 to45% at a temperature maintained at from 15° to 30° C. to form a finalcomposition having an A value in the range of 0.70 to 0.75.
 16. Aprocess according to claim 7 characterized in that the mixture of water,hydrogen peroxide and sulfuric acid in step 2 contains from 40 to 65%w/w sulfuric acid.